1. Field of the Invention
The present invention relates to an index quantitatively indicating shapes of kaolin particles for use as a material for manufacturing a cordierite honeycomb structure or the like, a method of measuring the index, and a method of manufacturing a cordierite honeycomb structure which is suitable for an application such as a catalyst carrier for purifying an exhaust gas from a car and which has a low thermal expansion coefficient and an excellent resistance to thermal shock.
2. Description of the Related Art
Heretofore, a cordierite honeycomb structure has been broadly used as a catalyst carrier for purifying an exhaust gas from a car. The most important required characteristic of the honeycomb structure for use in such application is a resistance to thermal shock, and is largely influenced by a thermal expansion coefficient of the honeycomb structure itself. That is, to improve a thermal shock resistance of the cordierite honeycomb structure, it is important to lower the thermal expansion coefficient of the honeycomb structure as much as possible. As means for lowering the thermal expansion coefficient of the honeycomb structure, attempts are made to develop micro cracks along a c-axis direction of a cordierite crystal (see Japanese Patent Publication No. 7-61892) and to control an amount of an iron content existing in a cordierite forming material (see Japanese Patent No. 2981107).
The above-described means are effective for lowering the thermal expansion coefficient of the cordierite honeycomb structure, but in an industrial field, there are demands for further lowering of the thermal expansion coefficient and stabilizing of the low thermal expansion coefficient of the cordierite honeycomb structure in the present situations.
Additionally, it is known that the resistance to thermal shock of the cordierite honeycomb structure is largely influenced by the thermal expansion coefficient of the honeycomb structure in a longitudinal direction (axial direction of a cell) and that the resistance to thermal shock is hardly influenced by the thermal expansion coefficient in a diametric direction. It is also known that the thermal expansion coefficient of the cordierite crystal in the c-axis direction is lower than that in an a-axis direction. Therefore, when the cordierite crystals are oriented so as to bring the longitudinal direction of the honeycomb structure in parallel with the c-axis of the cordierite crystals, the thermal expansion coefficient in the longitudinal direction can be lowered, and the resistance to thermal shock can be improved.
The cordierite honeycomb structure is manufactured by: blending particles of talc, kaolin, alumina and the like so as to obtain a theoretical composition of cordierite; forming the resultant cordierite forming material into a clay to extrude the clay; and firing the resultant molded article. In the course of the firing, the cordierite crystals grow from the kaolin particles (crystal particles) as nuclei so as to cross the c-axis of the kaolin particles at right angles. In consequence, if the crystals can be oriented so that the c-axis of the kaolin particles cross the longitudinal direction of the molded article at right angles during the extrusion molding, the cordierite crystals can be oriented as described above.
The orientation of the kaolin particles during the extrusion molding is largely influenced by the shapes of the kaolin particles themselves. In a case where the kaolin particles become flat, when the particles pass through a slit of a die during the extrusion molding, the particles are easily oriented as described above. The layered kaolin particles become flatter owing to delamination (interlayer peeling) as the number of the layers decreases. However, since there has not been a method of quantitatively measuring the shapes of the kaolin particles, it has been difficult to selectively use the kaolin particles having the above-described shapes suitable for lowering the thermal expansion coefficient as the material of the cordierite honeycomb structure.